This invention relates to tubing utilized in steam generating equipment, and, more specifically, to a method of forming variably rifled boiler tubes wherein the variable rifling is operative to provide controlled internal flow disruption within the tubes so as to thereby prevent stagnation of the steam bubbles that are formed during nucleate boiling.
It is well known to most that a major operating component of any conventionally constructed steam generating system is the boiler. Likewise, it is well known to all that it is in the boiler that the generation of steam is actually effected. In this regard, the aforesaid generation of steam is commonly accomplished as a consequence of the passage of water through a multiplicity of tubes, during which passage the water is sufficiently heated so as to cause it to change state, i.e., to change from a liquid to a vapor.
It is obviously important that boiler tube failure be avoided in such steam generating systems. Relative to the matter of boiler tube failure, one known cause thereof is overheating of the tubes. Furthermore, it is known that an inefficient transfer of heat through the tube wall to the water flowing therewithin can lead to the tubes overheating. The reference here to an inefficient transfer of heat through the tube wall is meant to encompass the situation wherein the accomplishment of the desired heat transfer process is impeded by the presence of so-called nucleate boiling, i.e., wherein stagnating steam bubbles that function in the nature of insulation impedes the passage of the heat through the tube wall to the water flowing therewithin.
To summarize, the condition which is sought to be avoided here in an effort to minimize the susceptibility of the tubes to become overheated as a result of nucleate boiling, is that wherein there exists within the tube a laminar flow of water or steam. As used herein, the term laminar flow is meant to refer to a streamline or viscous flow of the fluid axially of the tube. Namely, it is desired to effect the breaking up of such laminar flow in the tube.
As the water flows through the tube, the outer layer of the water, i.e., the layer of water in closest proximity to the inner wall of the tube, becomes heated by the heat being transmitted through the tube wall. As a consequence thereof, the outer layer of water changes to steam. During this process of changing to steam, the first change, which the outer layer of water undergoes, is the formation therein of steam bubbles. It has been previously mentioned hereinbefore that steam bubbles act as an insulation. Consequently, unless the steam bubbles, which are being formed in the outer layer of water are made to mix, they will, in essence, remain stationary, i.e., stagnate, and take on the attributes of an insulative film, thereby causing localized hot spots to develop along the tube wall. Moreover, such hot spots, in turn, can cause overheating of the tube, and ultimately lead to tube failure. Additionally, unless they are made to mix, the steam bubbles by virtue of their insulative capability will also function to prevent further heating of the core of water, which is passing rapidly through the center of the tube in the form of laminar flow, as described above.
Thus, from the preceding, it should be readily apparent that in order to achieve the rapid and efficient transfer of heat through the tube walls to the water flowing therewithin, there exists a need to provide some form of means that would be operative to effect the breaking up of the laminer flow of water through the tube. Namely, some such form of means is needed to effect the mixing of the outer layer of water and thereby also the steam bubbles entrained therein with the core of water flowing through the central region of the tube. One such form of means, which has been employed heretofore in the prior art, to achieve a controlled internal disruption of the flow of water through a boiler tube has involved the usage of ribbing, i.e., rifling, on the internal surfaces of the boiler tubes.
As regards the nature of the existing prior art teachings relating to the matter of providing boiler tubes with rifled inner wall surfaces, reference may be had by way of exemplification for purposes of obtaining a familiarity therewith to the following U.S. Pat. Nos. 3,088,494; 3,213,525; 3,272,961; 3,289,451 and 3,292,408. More specifically, U.S. Pat. No. 3,088,494, which issued to P. H. Koch, et al. on May 7, 1963, is directed to providing a vapor generating tube that has its interior wall formed with helical lands and grooves, which are proportioned and arranged in a particular predetermined fashion. In accord with another such exemplary prior art teaching, U.S. Pat. No. 3,213,525, which issued to W. M. Creighton, et al. on Oct. 26, 1965, is directed to a method of forming an internal rib in the bore of a tube wherein material is removed from the inner tube wall by means of a cutting operation to form the subject ribbing. A still further example of these prior art teachings can be found in U.S. Pat. No. 3,272,961, which issued to L. A. Maier, Jr., et al. on Sept. 13, 1966, and wherein a method and apparatus are taught for making ribbed vapor generating tubes and in accordance with which a rib is deposited on the inside surface of the tube by means of a welding process. U.S. Pat. No. 3,289,451, on the other hand, which issued to P. H. Koch, et al. on Dec. 6, 1966, is directed to a method and apparatus for forming internal helical ribbing in a tube wherein the internal ribbing is formed by means of a cold drawing operation. Finally, U.S. Pat. No. 3,292,408, which issued to J. R. Hill on Dec. 20, 1966, is directed to a method of forming internally ribbed tubes wherein the tube is provided with an asymmetrical helical groove so as to facilitate removal of the forming tool from the tube.
Notwithstanding the existence of the aforedescribed prior art teachings, there has nevertheless been demonstrated a need for a new and improved method of providing boiler tubes with a rifled interior surface. More specifically, the prior art methods that have been employed for this purpose heretofore have all been adversely characterized by the fact that they each suffer from certain notable disadvantages. For instance, those prior art methods and apparatus that involve the removal of metal from the interior wall surface of the tube to effect the formation thereat of helical lands and grooves suffer from the standpoint of cost. Namely, from a cost standpoint, effecting the formation of rifled tubes by means of methods involving the removal of metal from the inner tube surface constitutes a relatively uneconomical method of manufacturing the same. That is, it is relatively expensive to form helical grooves and lands in the inner wall surface of a tube by a metal removal process, and particularly in applications like the present one wherein there is a concomitant requirement that both the helical grooves and the helical lands each be of precise dimensions.
Another disadvantage from which prior art methods and apparatus for forming rifled tubes are known to suffer, and particularly such methods and apparatus wherein a metal deformation process is utilized involving the insertion of a member into the tube, is the difficulty that has been encountered in effectuating the removal of the member from the tube following completion of the metal deformation process. More specifically, in accord with such methods and apparatus, a member having a predetermined external configuration is inserted into the tube, and thereafter the tube is reduced in diameter such that the helical pattern that the member embodies is formed in the inner wall of the tube. In order to thereafter remove this member from the tube, it is necessary, because of the fact that the interior surface of the tube has been deformed so as to become essentially an exact complement of the member's external surface, to virtually unscrew the member from the tube to effect the removal of the former from the latter. Generally, speaking, the degree of difficulty that is encountered in effecting the removal of the aforesaid member from the tube is dependent on the length of the member, which has been inserted into the tube, and the relative extent to which the pattern formed on the inner tube wall is a true complement of the pattern formed on the external surface of the aforesaid member, i.e., to the extent the member and the tube take on the attributes of a threaded fastener that is threadedly engaged in a cooperating threaded nut.
Still another disadvantage from which prior art methods and apparatus for effecting the formation of rifled boiler tubes are known to suffer is the inherent inflexibility associated with the use thereof insofar as concerns effectuating variations in the configuration of the pattern that is to be formed in the surface of the tube inner wall. Namely, as noted above, nucleate boiling can lead to the development of localized hot spots that, in turn, can cause overheating and ultimately boiler tube failures. To minimize the establishment of such localized hot spots in boiler tubes stemming from the existence of nucleate boiling, it has been proposed by the prior art to provide ribbing, i.e., rifling, on the inner wall surface of the tube. Unfortunately, however, the methods and apparatus known in the prior art here to date for effectuating the making of such rifled tubing render it virtually impossible to enable variations in pattern configuration to be implemented for purposes of compensating for variations in the location of potential hot spots along the inner walls of the tubes. That is, existing methods and apparatus are limited to the utilization of patterns of fixed configuration, such that each boiler tube irrespective of the location it occupies in the boiler, i.e., its relative exposure to external sources of heat, is necessarily provided with the same pattern of rifling, even though from a heat transfer standpoint, it would be desirable to vary the pattern as between locations within the same tube, as well as between different tubes in the same boiler.
To summarize, a method of providing boiler tubes with a rifled inner surface is desired that would embody, among others, the following characteristics. First, there is desired a method that, from the standpoint of cost of manufacture, is relatively economical to utilize. Secondly, a method is desired, which, from the standpoint of ease of usage, is relatively simple to employ. Thirdly, a method is desired, which is characterized by the fact that it enables a selection to be made amongst a diversity of patterns of rifling that are capable of being employed in a boiler tube inner wall, such that the use of a given rifled pattern can be predicated upon the fulfillment of the heat transfer requirements, which are particular to each individual boiler tube.
It is, therefore, an object of the present invention to provide a new and improved method of making boiler tubes wherein the latter are provided with means operative to cause a controlled internal flow disruption to be effected therewithin.
It is another object of the present invention to provide such a method of making boiler tubes wherein the latter are provided with inner tube surfaces that are rifled.
It is still another object of the present invention to provide such a method of making boiler tubes, which is operative to enable boiler tubes to be provided that are variably rifled.
A further object of the present invention is to provide such a method of making variably rifled boiler tubes wherein the pattern of rifling desired to be formed in the tube inner surface is established by detachably wrapping a wire-like member around the circumference of a hardened mandrel.
A still further object of the present invention is to provide such a method of making a variably rifled boiler tube wherein a hardened mandrel having a wire-like member detachably wrapped around the circumference thereof to establish a pattern thereon is inserted into the tube whereupon a reduction in tube diameter is effected causing the wire-like member to become embedded in the inner wall of the tube.
Yet another object of the present invention is to provide such a method of making a variably rifled boiler tube wherein, alternatively, either the hardened mandrel may be removed from the boiler tube leaving the wire-like member embedded in the tube inner wall so as to be operative in the manner of rifling, or both the hardened mandrel and the wire-like member may be removed from the boiler tube, in which case the impression left by the wire-like member in the inner surface of the tube is operative as rifling.
Yet still another object of the present invention is to provide such a method of making a variably rifled boiler tube which is advantageously characterized by the fact that it is relatively inexpensive to utilize, relatively simple to employ, and is extremely flexible insofar as concerns the variety of different patterns of rifling that can be formed therewith in boiler tubes.